The present invention generally relates to current drive circuits and, more particularly, to a current drive circuit of a current mirror type. A current mirror circuit is often used to feed a desired current to a load. A current mirror circuit generally used has the following structure. The gates of first and second transistors are connected to each other and so are the sources of the transistors. The sources of the transistors are grounded and the gates are connected to the drain of the first transistor. A target load is connected to the drain of the second transistor.
A reference current is fed to the drain of the first transistor and a drive current proportional to the reference current is fed to the load connected to the drain of the second transistor. The ratio between the reference current and the drive current, i.e. the mirror ratio, is determined by the ratio between source-drain currents of the first and second transistors. The source-drain current IDS is proportional to the channel width W of a transistor and inversely proportional to the channel length L thereof. Generally, the source-drain current is determined by the ratio W/L.
The ratio between the reference current and the drive current is determined by the ratio W/L of the first and second transistors. However, such a definition is based on an assumption that source-drain voltages VDS of the transistors are identical. Strictly speaking, it is known that the source-drain current IDS of a transistor is proportional to (VGS−Vth)2*(W/L)*(1+λVDS), meaning that IDS is slightly affected by VDS. λ indicates a channel length modulation coefficient, VGS indicates a gate-source voltage and Vth indicates a threshold voltage. Accordingly, even when W/L is designed properly, an accurate drive current is not obtained when VDS of one of the transistors is different from an ideal value.